Obi Wan Kenobi was no regular Jedi. He was probably one of the best of his time, only second to Yoda and maybe Mace Windu (dueling that is). He was a master duelist and even better with the force. And he was used as a tool. Darth Sidious detected his potential and wished to utilize it and make him his apprentice. Vader wasn't necessary to throw over the Republic.